A significant proportion of the HL-A histocompatibility antigens present in the American black population are either not defined or only poorly defined by presently available reagents. The purpose of the proposed research is to improve the definition of HL-A antigens in black persons by analysis of the reaction patterns of families in serological tests and by obtaining and characterizing new HL-A reagents. Blood samples obtained from black families will be tested for histocompatibility antigens using lymphocytotoxic methods, and serum samples obtained from black multiparous women will be screened for new lymphocytotoxic antibodies. New antigens may be identified by the black multiparous sera and by a systematic study of reagents currently available. HL-A cross-reacting antigen groups, defined primarily through studies in Caucasian populations will be studied in the black population by various lymphocytotoxic methods, by absorption and elution studies, and by blocking studies. Patterns of HL-A inheritance observed in participating families and serum obtained from multiparous women will help to improve the definition of HL-A antigens in black persons. This improvement in HL-A typing will allow better matching of black donors and recipients in clinical transplantation and will provide serological markers for studies of other immunologically important loci, such as the MLC locus, which are genetically linked to HL-A.